Image-frequency suppression system



Feb. 6, 1940. D. v. SINNINGER IMAGE-FREQUENCY SUPPRESSION SYSTEM Filed Jan. 19. 1939 INVENTOR my/a/r k Jaw/N6512- WWG. 6am

ATTORNEY Patented Feb.

UNITED STA'l'ES- "V-PA'IIIENT OFFICE 2,188,964 IMAGE-FREQUENCY sUrranssIoN SYSTEM Dwight V. sinninger, Chicago, Ill., asslgnor to Johnson laboratories, Inc., Chicago, 111., a corporation of Illinois screams.

This invention relates to high-frequencycircults and more particularly to those employed in radio receiving systems of .the superheterodyne type, where the signal passes throughone or more resonant circuits tuned to the signal frequency, is modulated with locally produced oscillations, then is demodulated and amplified at an intermediate frequency, and finally is demodulated again and rendered audible. The part of the system through which the signal passes before any change in frequency occurs usually includes an antenna circuit and one or two resonant circuits with or without afthermionic amplifying 'vacuum tube, and may be termed the preselector.

The adjacent-channel selectivity obtainable in a superheterodyne system'is largely due to the efficiency of the intermediate-frequency amplifier which, being operated at a fixed frequency,

may be designed to have a desired selectivity characteristic. The preselector of the receiving system serves to collect the desired signal, to

aid in the rejection of undesired signals in adjacent channels, and to substantially reject sig- 5 nals of certain other frequencies to which a superheterodyne receiver tends to respond.

When the usual superheterodyne radio receiver is tuned .to' a desired signal, some response is likely to be obtained from a second signal which diflers from the frequency of the desired signal by twice the intermediate frequency of the receiver. The undesired signal may be called an image-frequency signal. For example, if thedesired signal has a frequency of 600 kilocycles and the intermediate frequency of the receiver is 465 kilocycles, a. signer having a frequency of 600+ (2x465) or 1530 kilocycles will be an imagefrequency signal. The ra'tio'of the input voltage ct image frequency required to produce -a given 40 output, to the signal voltage required "for the same output, is called the image-frequency ratio. A feature of the present invention'is to substantially raise the value of this ratio by the proper selection and arrangement o1 the circuit components.

My invention is intended for use in connection with resonant circuits which are tunedover a range of frequencies by inductance variation. Such a system is the one disclosed by Polydoroff in United States Patent No. 1,940,228, in which a resonant circuit having an inductance coil and capacitor is adjusted-over a range of frequencies 'by movement of a compressed comminuted ferromagnetic core relative to the inductance, coil. This method of tuning is commonly called perme- Application January 19, 1939, Serial No. 251,881

ability tuning. An improved form of such a system is disclosed by Schaper in United States Patent No. 2,051,012. Both Polydoroif's original system and Schapers improved system readily cover an adequate range of frequencies and may easily 5 be ganged to provide, multiple unit systems. Permeability tuning is especially adapted for use in the preselector of a superheterodyne radio receiver because its use permits the preselector to be designed so as to provide substantially .uni- 10 form gain and selectivity throughout the frequency range, thus overcoming the chief cause of non-uniformity of performance in this type of receiver.- 4

Although my invention may be advantageously 15 employed in superheterodyne radio receivers hava ing more than one variably tuned resonant cirplished in the manner to be described below. 30 i In accordance with my invention, image-frequency interference is minimized by providing a high-impedance path or trap circuit forimagefrequency signals between the antenna or other collector and the input electrode ofthe first vacuum tube of the receiver. Since the preselector is tunable over a range of frequencies, the imagefrequency trap circuit must also be tuned over" a different but properly related range of frequencies, and this must be accomplished auto- 4 matically and simultaneously.

In the superheterodyne type of receiver, the intermediate frequency is' usually lower than the lowest desired'signal frequency, and the local oscillator frequency must differ from the'reso-u nant frequency of the preselector by a practically constant amount. The difference is equal to the intermediate frequency,the frequency of the cacillator usually being higher than that of the preselector. so that the oscillator must be tun- 5o able over a higher but narrower range of frequencies than'that covered by the. preselector. The image frequency, which in this case is higher than the signal frequency by twice the intermediate frequency, falls within a range which is 5 6 relative to coil ii.

still higher and narrower. example, ifa receiver has an intermediate frequency of 465 g proximately three-to-one frequency range and tune. a second resonant circuit over a higher butnarrower frequency range having a ratio of maximum to minimum frequency of approximately 60 percent that of the first resonant circuit.

The invention will be better understood if reference is made to the accompanying drawing, in which:

Fig. 1 is a schematic diagram of the preselector portion of a radio receiver embodying one form of my invention;

Fig. 2 is a schematic diagram of .a modified form of the preselector of Fig. 1; and

Fig. 3 is a schematic diagram of another modi-. fication of the preselector of Fig. 1.

Ref ring to Fig. 1, the antenna 8 is connected to gr1 nd through capacitors 2 and 3 in series. Varia le inductance device 6, comprising inductance coil 5 and movable ferromagnetic core 6, is shunted by capacitor 7, and by capacitors 3 and 8 in series, to form a resonant circuit 0. The high-potential terminal of resonant circuit is connected to control-grid it of vacuum tube M by means of capacitor I 2. Direct-current bias potential is supplied to control-grid it by grid leak l3. Capacitor-3 is shunted by resistor is to provide a low-impedance path to ground for disturbing audio-frequency currents induced in antenna I. In operation, resonant circuit 9 is tuned over a range of' frequencies by movement of core .This circuit may be adjusted to resonance at a desired frequency near the high-frequency end of its tuning range, with core 6 substantially withdrawn from coil d, by

. means of adjustable capacitor 8.

Inductance device 1 and capacitor F also form a resonant circuit I which is tuned, by movement of core 6 relative to coil 6, over a range of frequencies higher than that covered. by resonant circuit -9. By properly choosing capacitor i with respect to capacitors 3 and B, resonant circuit It may be made to resonate exactly at the image frequency which corresponds with any particular resonant frequency of resonant cirouiti, and to resonate approximately at the image. frequency corresponding to other frequencies within the tuning range of resonant circuit 9. An

examination of Fig. 1 will show that resonant circuit I5 is effectively in series between antenna l and control-grid E0 of vacuum tube I I, so that a high-impedance path is provided for signals of image frequency. Signals of the desired frequency, however, are not appreciably blocked, since resonant circuit l5 has a relatively low impedance at this frequency. It will be understood that capacitor 2 is not essential and may be eliminated if desired.

Referring to Fig. 2 of the drawing, the circuit arrangement is similar to that of Fig. 1 and like components are designated by like reference numerals. Ferromagnetic core B'in this case has" appreciable surface conductivity and is effectively grounded, either conductively or through a low-impedance resistive or capacitive path. The capacitance between the core and the highpotential portion of coil 5 is represented by dottedcapacitor 86, the fact that the curved arrow passes through it indicating that its capacitance varies as core 6 movesrelatively to coil 5. The increase in capacitance it during the latter half of the movement of core 6 as it enters coil 5 accelerates the lowering of the resonant frequency of resonant. circuit 9 in such a way that the resonant frequency of resonant circuit l5 remains much closer to the image frequency which corresponds to each position of core 6 with respect to coil 5. Thus in accordance with my invention I utilize the core simultaneously to tune two resonant circuits over two different frequency ranges and to maintain a practically constant frequency difference between their resonant frequencies over a portion of the range of adjustability. I

In Figs. 1 and 2, it will be understood that capacitor i may be replaced by a predetermined value of distributed capacitance built into in ductance coil 5 in the well-known manner. It is one of the features of my invention to thus control andutilize the natural resonant frequency of the inductance coil if desired.

Referring to Fig. 3 of the drawing, the circuit arrangement is similar to that of Fig. 2 with one exception, like components being designated by like reference numerals. Inductor ii is connected in series with capacitor ll across inductance coil 5., The presence of inductor i7 lowers the rate of change of the resonant frequency of resonant circuit it during the latter half of the movement of core 6 as it is withdrawn from coil 5 in such a manner that it'remains much closer in accordance with Fig. 3 and intended for use with an intermediate frequency of 465- kilocycles, the following constants are employed:

Capacitor 2 (if used) 250 micromicrofarad Capacitor S 500 do. Inductance coil 5 .125 microhenriesp Gapacitor 22 micromicrofarads Gapacitor 8 18 do, Resistor it .15,000 ohms Inductor Ill 7 microhenries The average image-frequency ratio over the signal-frequency range-of 540 to 1600 kilocycles with this particular arrangement is approxmately 5000.

It will be understood that where in the speci-,

fication I state that one circuit is "tunable over a narrower range of frequencies than a second circuit, reference is made to the fact that the ratio of the'width of the band to thefrequency at its center is less in the case of "the first circuit. Where it is stated inthe specification that one circuit is tunable over a higher range of fre quencies than a second circuit, reference is made to the fact that the minimum frequency'of the first circuit is higher than the minimum frequency of the second circuit and that themaiiimum frequency of the first circuit is high'erftha the maximum frequency of theseco'ndcircuit.

Having thus described-my invention, I claim:

1. An image suppression system for a super- 'heterodyne radio receiver having an antenna variation respectively to a signal frequency andto the corresponding image frequency.

2. An image suppression system for a superheterodyne radio receiverfiiavihg an antenna and-a vacuum tube with a control electrode, in-

eluding a first resonant circuit a portion of which is connected between said control electrode 'and ground, and a second resonant circuit in series between said antenna and said control electrode, said resonant circuits including in common a variable inductance device comprising an inductance coil and a ferromagnetic core movable relatively thereto and being tunable simultaneously over substantially different frequency ranges by motion of said core.

3. An image suppression system for a superheterodyne radio receiver having an antenna and a vacuum tube with a control electrode, including a first resonant circuit a portion of which is connnected between said control electrode and ground, and a second resonant circuit in series between said antenna and said control electrode, said resonant circuits including in common a variable inductance device comprising an inductance coil and a ferromagnetic core movable relatively thereto, said resonant circuits being simultaneously tuned by motion of said core respectively to a signal frequency and to the corresponding image frequency.

4. A preselector system for a superheterodyne radio receiver, including a variable inductance device comprising an inductance coil and a movable ferromagnetic core, first and second capacitors connected with said inductance device to secure resonance within a first range of frequencies by movement of said core, and a third capacitor in shunt with said inductance device to secure resonance within a second range of frequencies by movement of said core, said capacitors being so chosen that any frequency of said second range is higher than the corresponding frequency of said first range by a practically constant amount throughout said core movement.

5. A preselector system for a superheterodyne radio receiver, including a variable inductance device comprising an inductance coil and a movable ferromagnetic core', first and second capacitors in series shunted across said inductance device to secure resonance within a first range of frequencies by movement of said core. and'a third capacitor in shunt with said inductance device to secure resonance within asecond range of frequencies by movement of said core, said capacitors being so chosen that any frequency of said second range is higher than the corresponding frequency of said first range by a practically constant amount throughout said core movement.

6. In a superheterodyne radio receiving system, an antenna, a vacuum tube having an input electrode, and elements including a variable inductance device comprising an inductance coil and a movable ferromagnetic core, a first resonant circuit including said inductance device and tunable thereby over a first rangeof frequencies connected between said antenna and ground, and

a second resonant circuit including saidinductance device and tunable thereby over a .second frequency range in series between said antenna and said input electrode, said elements being so proportioned that said resonant circuits are maintained at a practically constant frequency difference during movement of said core.

7. A preselector system for a superheterodyne radio receiver, including a variable inductance device comprising an inductance coil and a movable ferromagnetic core, said core having surface conductivity and being effectively grounded,

. first and second capacitors associated with said inductance device to secure resonance. within a first range of frequencies by movement of said core, and a third capacitor in shunt with said inductance device to secure resonance within a second range of frequencies, said capacitors being so related to the capacitance between said coil and said core that the resonant frequency of said second resonant circuit is higher than that of said first resonant circuit by a substantially constant amount throughout said core movement.

8. A preselector system for a superheterodyne radio receiver, including a variable inductance device comprising a first inductance coil and a movable ferromagnetic core, said core having surface conductivity and being effectively grounded, first and second capacitors associated with said inductance device to secure resonance within a first range of frequencies by movement of said core, and a series-connected second inductance coil and third capacitor in shunt with said inductance device to secure resonance within a second range of frequencies by movement of I' range by a substantially constant amount throughout said core movement.

DWIGHT V. SINNI'NGER.

than the corresponding frequency of said first 

